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Auto unseal removes the manual overhead of key-shares at startup by delegating master key encryption and decryption to an external Key Management Service (KMS) or Hardware Security Module (HSM). This streamlines Vault operations, improves reliability, and ensures high availability.

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How Auto Unseal Works

  1. Master Key Encryption
    Vault encrypts its master key with a key stored in an external KMS/HSM instead of splitting it into manual key-share shards. Supported providers include cloud KMSes (AWS KMS, Azure Key Vault, Google Cloud KMS) and on-prem HSMs.
  2. Storage Backend
    The encrypted master key is persisted in your chosen storage backend (Consul, etcd, S3, etc.).
  3. Service Startup / Restart
    When Vault boots or restarts, it:
    • Reads the encrypted master key from storage.
    • Calls out to the configured KMS/HSM to decrypt the master key.
    • Uses the decrypted master key to unlock the data encryption key (DEK).
    • Loads the DEK into memory to access and serve secrets.
  4. Automatic Unseal
    No operator action is needed. As soon as Vault initializes, it retrieves and decrypts its master key via the KMS/HSM and completes the unseal process automatically.
Auto unseal works with both open-source and Enterprise Vault. Since Vault 1.0, this feature is available in the open-source edition.

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Supported Auto Unseal Providers

ProviderSeal StanzaDocumentation
AWS KMSseal “awskms”https://www.vaultproject.io/docs/secrets/aws#auto-unseal-using-aws-kms
Azure Key Vaultseal “azurekeyvault”https://www.vaultproject.io/docs/secrets/azure#auto-unseal-using-azure-kv
Google Cloud KMSseal “gcpckms”https://www.vaultproject.io/docs/secrets/gcp#auto-unseal-using-gcp-kms
AliCloud KMSseal “alicloudkms”https://www.vaultproject.io/docs/secrets/alicloud#auto-unseal
On-prem HSM (PKCS#11)seal “pkcs11”https://www.vaultproject.io/docs/secrets/pkcs11#auto-unseal-using-pkcs11-hsm

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Configuration Example: AWS KMS

Add the seal stanza beneath your global configuration to enable AWS KMS auto unseal: 
storage "consul" {
  address = "127.0.0.1:8500"
  path    = "vault/"
}

listener "tcp" {
  address         = "0.0.0.0:8200"
  cluster_address = "0.0.0.0:8201"
}

seal "awskms" {
  region     = "us-east-1"       # AWS region where the KMS key resides
  kms_key_id = "arn:aws:kms:us-east-1:123456789012:key/abcd-1234" 
}

api_addr = "https://<VAULT_IP>:8200"
ui       = true
  • seal “awskms”: Configures AWS KMS as the unseal mechanism.
  • region: The AWS region where your KMS key exists.
  • kms_key_id: The full ARN or key ID of the KMS key used to encrypt/decrypt the Vault master key.
Do not commit your kms_key_id (or any credentials) into source control. Use environment variables or a secrets management workflow to inject sensitive data.
You can swap out "awskms" for any of the other supported providers by updating the seal stanza accordingly.

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